In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods. Each fuel rod contains a stack of pellets of a nuclear fuel arranged in a cladding tube, a column of extruded fuel cylinders or an uninterrupted column of vibration-compacted powdered fuel. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly.
Together with a plurality of other fuel assemblies, the fuel assembly is arranged in a core. The core is immersed in water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby, in a boiling water light-water reactor, part of the water is transformed into steam. The percentage of steam increases towards the top of the fuel assembly. Consequently, the coolant in the lower part of the fuel assembly consists of water whereas the coolant in the upper part of the fuel assembly consists both of steam and of water. This difference between the upper and lower parts gives rise to special problems which must be taken into consideration when designing the fuel assembly.
This problem can be solved by achieving a flexible fuel assembly which, in a simple manner, may be given a shape in which the upper part of the fuel assembly differs from the lower part thereof such that optimum conditions can be obtained. A fuel assembly for a boiling water reactor with these properties is shown in International patent document PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may, in a simple manner, be given a different design in its upper and lower parts.
Also in a light-water reactor of pressurized-water type, it may be desirable to design the fuel assemblies such that each fuel assembly comprises a plurality of fuel units stacked on top of each other. As described above, each one of the fuel units then comprises a plurality of fuel rods extending between a top nozzle and a bottom nozzle. A fuel assembly for a pressurized-water reactor, however, comprises no fuel channel.
One factor which must be taken into consideration when designing such fuel units with a length on the order of 300-1500 millimeters is that fission gases are formed during nuclear fission. In addition, the column of fuel pellets expands because of the heat generated in the fuel pellets. To take care of the fission gases and the thermal expansion of the column of fuel pellets, a relatively large space, an axial gap, is normally formed above the uppermost fuel pellet in the cladding tube in known full-length fuel rods, that is, fuel rods with a length on the order of 4 meters. The axial gap has a length on the order of 200-300. The fission gases may thus diffuse to this axial gap and the column of fuel pellets may expand into this gap.
Another factor which must be taken into consideration when designing axial gaps is that the temperature of the cladding tube in this region is lower than in the rest of the cladding tube since no fuel pellet is arranged in the axial gap. A problem which may arise as a result of this is that hydrogen formed, inter alia, by corrosion of the cladding tube, which is of a zirconium-based alloy, and is taken up thereby, diffuses into this colder region. In the event that the concentration of hydrogen becomes too high in this region, hydrides are formed in the cladding material and cause embrittlement thereof. In a serious case, the cladding tube may burst and fissionable material may enter into the cooling water. The same type of problem may also arise in the regions between the pellets, that is, where a lower end of a fuel pellet makes contact with an upper end of an adjacent fuel pellet, and in the region between two fuel units stacked on top of each other. The risk of embrittlement due to too high a concentration of hydrogen increases, to a certain limit, with the size of the axial gap.
Released fission gas contributes to the temperature in the axial gap decreasing further. This is due to the fission gas deteriorating the thermal conductivity of the gas which is present in the axial gap. The same thing applies to the gas which is present in the gap between the fuel pellets and the cladding tube, in which case the difference in temperature between the outer surface of the pellets and the inner surface of the cladding tube increases.
It is known to reduce the release of fission gas in different ways. One such way is to provide one or more of the fuel pellets with through-holes in their axial directions. In this way, the temperature in the fuel pellet is lowered whereby the release of fission gas is reduced and the axial gap may be reduced. In this case, the axial gap may be limited on the order of a few millimeters in a rod with a length on the order of 300 millimeters, up to a few tens of millimeters for longer rods, to allow the thermal expansion of the column of fuel pellets. A disadvantage of pellets provided with through-holes is that they are complicated to manufacture. For that reason, it is desirable to arrange axial gaps in the fissionable material.
Still another factor which must be taken into consideration when designing axial gaps in a fuel rod is that local power peaks arise here. The power peaks arise due to the moderation in this region, where fissionable and neutron-absorbing material are missing, being very good. This results in the power in the pellets adjoining the axial gap becoming very high, that is, a power peak arises. The power peak grows with the size of the axial gap.
The object of the present invention is to provide a fuel assembly with a plurality of short fuel units with fuel rods formed with axial gaps in the fissionable material adapted to give rise to small power peaks only.